1. Field of the Invention
This invention is directed to an improvement in the art of forming foamed and unfoamed molded plastic articles.
2. Description of the Prior Art
"Additives" throughout this Specification and claims are defined to mean substances or materials added to thermoplastic resins. Examples of such additives are flame retardants, U.V. stabilizers, blowing agents, colorants, etc. Examples of thermoplastic resins useful in this invention are polyethylene, polypropylene, polystyrene, polyvinyl chloride, copolymers thereof and the like. In general, any injection moldable thermoplastic resin can be used. The following, for purposes of discussion, considers prior art relating to blowing agent additives.
Prior methods and apparatus for adding blowing agents to a plastic resin to form solid skin and foamed core (SSFC) articles are exemplified in the patents to De Vita, U.S. Pat. No. 3,746,492 (1973), D. F. Oxley, U.S. Pat. No. 3,751,534 (1973), P. J. Garner and D. F. Oxley, U. K. Pat. No. 1,156,217 (1969), and R. G. Angell, Jr. U.S. Pat. No. 3,436,446 (1969). In each of the above cited art, a blowing agent is introduced into a thermoplastic material to form a foamable plastic mixture. This mixture is then accumulated under pressure in an accumulator or an injector. An injector is used throughout this Specification and claims to mean a device capable of both accumulating and injecting under pressure molten plastic into a mold. Some examples of injectors are the accumulators disclosed in the above cited art and reciprocating screw or plunger injection molding machines such as sold by Cincinnati Milacron.
One known method for injecting foamable and unfoamable plastic material alternately into a mold involves the use of two coupled injection systems, one for foamable and the other for unfoamable plastic materials. These systems are arranged to inject alternately through a single nozzle. Another known method uses a single injection system containing discrete layers of foamable and unfoamable plastic material. Upon injection, each discrete layer enters the mold separately, thereby injecting alternately in a single injection stroke foamable and unfoamable plastic into a mold.
There are several disadvantages inherent to these known methods. In the case where two injection systems are required, there are the substantial equipment costs and the need during filling for back-pressures in one of the injector systems to prevent premature foaming. Similarly, in the case where a single injection system employs discrete layers of foamable and unfoamable plastic, back-pressures on the foamable plastic material is required prior to injection in order to prevent premature foaming. In general, process control is more difficult where back-pressures are necessary. Further where an additive other than a blowing agent is added to a molten plastic before the molten plastic material is accumulated in an accumulator, degradation of said material often occurs to such a degree as to preclude the use of many otherwise useful additives.
H. L. Maiocco, U.S. Pat. No. 3,817,675 (1974) discloses an apparatus for introducing a measured quantity of colorants into a plasticized plastic stream just downstream of an extruder and just upstream of a mixer. This apparatus is similar to De Vita ('492) in that in De Vita a blowing agent instead of a colorant is introduced just downstream of an extruder and just upstream of a mixer.
Maiocco ('675) as disclosed has several limitations. There is no provision for selectively adding colorants to a plasticized plastic. There is no discussion concerning the addition of blowing agents, U.V. stabilizers or flame retardants. There is not disclosed the substitution of an injector for an extruder. That there is no means disclosed for selectively adding colorants makes a process of selectively introducing additives either into the skin or core of a molded plastic article very difficult to achieve. Since the substitution of an injector with its accumulation zone for an extruder is not disclosed, there is not taught a practical means for injecting in a periodic flow the quantities of a plasticized plastic often required during an injection molding cycle.